We are studying the mechanisms by which human T-cell leukemia virus type 1 (HTLV-1) enters and exits cells and replicates its genome in between. We are also investigating the effects of viral regulatory proteins on cell and virus gene expression pathways that underlie HTLV-1 pathogenesis. We have developed infectious molecular clones of HTLV-1 and virus vector systems to study individual steps in the viral infectious cycle. We found that HTLV-1 replication is restricted at a post-entry step in the infection process and we are currently investigating the basis for this block. The HTLV-1 vector system was used to determine the susceptibility of HTLV-1 to various nucleoside reverse transcriptase inhibitors. We have begun to characterize the physical and biochemical properties of the HTLV-1 reverse transcriptase protein. The interactions between cell and virus proteins that mediate the assembly and release of virus particles from cells are under investigation. Associations of HTLV-1 structural proteins with cellular endosomal sorting pathways have been identified. With respect to virus gene expression, we have examined the expression levels of putative regulatory proteins encoded in open reading frames in the 3? end of the virus genome. The products of these genes have not been detected in virus infected cells but there is strong evidence that they are produced in vivo. We examined the levels of expression of the alternatively spliced mRNAs that encode these proteins using real-time quantitative RT-PCR methods and splice site-specific primers. Similar PCR strategies were used to examine the control of lentivirus alternative splicing in vitro and in vivo. Changes in T-cell gene expression after HTLV-1 infection have been investigated using DNA microarray technology, RNase protection and real-time quantitative RT-PCR. We found that HTLV-1 transformed T-cells express specific type 2 cytokines including IL-9, IL-13 and IL-5. The HTLV-1 induced expression of IL-13 was investigated in detail in cells infected in vitro and in lymphocytes from infected individuals. The expression of IL-13 in HTLV-1 infected cells may have important implications for the pathogenesis of HTLV-1 associated disease.